Use lambdas instead of QSignalMapper

Summary: QSignalMapper is obsolete and code with lambdas is shorter and clearer.

Reviewers: Ark, elvisangelaccio

Reviewed By: Ark, elvisangelaccio

Subscribers: anthonyfieroni, kde-utils-devel, Ark

Tags: Ark

Differential Revision: https://phabricator.kde.org/D15249